The present invention relates to methods and apparatus for generating and transmitting agile frequency test signals, such as frequency hopped signals, to receivers under test and particularly to provide test signals that enable testing of receiver performance beyond nominal receiver performance.
Agile frequency signals are signals that have rapidly changing frequency and time characteristics and include, for example, frequency hopping signals. Frequency hopping is a form of spread-spectrum signaling where, for short instances of time, relatively narrowband signals are transmitted as short bursts with the carrier frequency for each burst tuned to a different one of a set of carrier frequencies than the ones of the carrier frequencies used for the previous burst and the next burst. The sequence of frequencies that is used for a sequence of bursts is known as the hopping sequence. The carrier frequency transmission at any particular instant of time for one burst is therefore different than the carrier frequency transmission at the previous instant of time for the previous burst and similarly is different than the carrier frequency transmission at the next instant of time for the next burst. While the bandwidth for any particular burst may be narrow, the bandwidth for the whole set of frequencies in the hopping sequence can be very large. Typically a frequency hop system hops over a bandwidth many times the bandwidth of the individual hop signal bandwidth. Bluetooth for example has a 1 MHz signal bandwidth and hops over 80 MHz. Some military radios have a 25 kHz signal bandwidth with thousands of hop frequencies covering over 50 MHz. The frequency hoppers in use today hop over at least 8 times the bandwidth of the signal bandwidth.
Frequency hopping systems with changing frequency transmissions have a number of advantages over the fixed frequency transmissions of non-hopping systems. If a particular hop frequency, in the set of frequencies used in a hopping sequence, happens to include a frequency that is regularly occupied by another interfering radio signal, the frequency hopping system detects the occupied status and functions to retransmit the burst of data at a different frequency. Also, the frequency hopping system detects the regularly occupied frequencies for any particular installation and reestablishes a hopping sequence that excludes the occupied frequency from the set of frequencies in the hopping sequence.
Frequency hopping systems are more secure than fixed frequency systems because the interception of frequency hopped signals is significantly more difficult than interception of fixed frequency signals, particularly when the hopping sequence is not known in advance. If a communication protocol is intended to be secure, such as in military and other secure environments, the hopping sequence and other protocol, specification and standards information is not published and is changed from time to time to support secure operation.
In any environment, the characterization of radios and radio wave signals for frequency hopped systems is difficult because they operate and function over broad bandwidths and because each burst at a particular frequency is of relatively short duration. The characterization of signals for frequency hopped systems is even more difficult when done in a secret environment where the protocol, specification, standards, hopping sequence and other characterizing information is not fully known in advance. A secret environment is common since manufacturers and users of frequency hopping systems often wish to maintain their protocols, specifications, standards and hopping sequences confidential and unpublished.
As the complexity of radios increases, the ability to adequately test the radios becomes more difficult. One common test procedure employs a “golden radio”. A golden radio is a radio that operates “nominally”, where “nominally” is loosely defined to mean an “average”, a “mean” or and “expected” operation. Each radio under test (test radio) is tested to ensure acceptable communication with the golden radio. If a test radio communicates well with the golden radio, then the test radio is accepted and if not, the test radio is rejected. This golden radio test method is limited because in actual use, radios will communicate with other radios that do not behave nominally. Large failure rates (for example, as high as 30%) often result in actual use when only golden radio testing is employed.
Other common test procedures employ test equipment in the form of signal generators for generating test signals for testing radios where the test signals are selected to have nominal values determined, for example, from specifications established for the transmit and receive characteristics of the test radios. Test equipment such as the Agilent E4438C, Agilent E8267C and the Tektronix SMIQ series can generate transmit signals to test standard wireless communications system such as 2, 2.5 and 3G cellular telephone systems and 802.11 wireless networks. The Tektronix SMIQ can generate a Bluetooth frequency hopped signal over a frequency band, but the band is narrower than the specified 80 MHz available for Bluetooth. All of these signal generators use symbols that are either random in nature or specified in the standard to produce a known test signal. These test systems do not achieve satisfactory testing since they do not adequately test the range of operation actually encountered by radios in a real environment where many radios having some non-nominal characteristics (all having passed nominal tests, however) fail to communicate satisfactorily.
Because of the difficulty of testing frequency hopping radios, the above and many other systems employ “hop-in-place” analysis where the test does not occur with frequency hopping or even if some hopping occurs, the hopping is not permitted to extend over the full hopping bandwidth available. These systems, therefore, do not adequately test frequency hopping radios.
It is desired to test the radios with transmitted signals having signal parameters that cover the range of the specified values and tolerances of the radios with accurate control over the signal attributes including sequences, symbols and parameters.
Various signal simulators have been proposed that deal with the transmission problems encountered in communications. U.S. Pat. No. 6,438,357 simulates the path loss encountered in the transmission of cellular telephone signals. U.S. Pat. No. 6,307,879 provides a method of compensating for distortion in the radio transmission process. U.S. Pat. No. 6,058,261 simulates Doppler, delay, multipath and delay spread encountered in the transmission process. None of these patents discuss changing the fundamental transmitted signal parameters to represent the range of parameters that are present when many different radios are communicating in actual operation in a non-test environment. While changes in the carrier frequency have been implemented by simulating Doppler, such a change is limited and does not allow change of frequency over the range of operation permitted by the specification for the radio. In the case of frequency hopped signals, the inadequacy is even greater since the carrier frequency changes for each hop.
U.S. Pat. No. 6,128,474 tests the diversity reception of a multiple antenna radio. U.S. Pat. Nos. 6,243,576 and 6,112,067 discuss a standard stimulus/response test system where the system transmits a known signal to a test device, such as a cellular telephone, and receives the response signal transmitted by the test device to ensure the test device properly received and processed the test signal. While these systems test against the known signal, such systems do not test the range of signals likely to be encountered in a real environment.
Other systems simulate signals for testing radars. U.S. Pat. No. 6,075,480 covers a system to simulate Doppler shift on complex radar signals. U.S. Pat. No. 5,117,230 records and plays back radar signals using signal processing to simulate the target encounter. U.S. Pat. No. 4,168,502 simulates a radar signal digitally to simulate a range of target velocity and acceleration. None of these patents deal with changing the signal parameters over the range of the radio specification other than those unique to the changes encountered in the transmission/reception process and hence they do not deal with the actual parameter variations from radio to radio in real communication systems.
The known test systems provide limited testing of radios without adequately testing the range of variables likely to be encountered in communications systems and therefore, they are not fully adequate for the communication industry.
Accordingly, in order to meet the demands of the communication industry, improved methods and apparatus are needed for generating agile frequency signals for broadband systems.